The development of the USB (Universal Serial BUS, universal serial bus) technology makes connections and communication between various external devices and computers become very convenient and fast. A common plug-and-play device needs an active USB interface to perform data transmission and supply power. In order to prevent a connection failure or a burned USB port due to an external device extracting excessive currents from the USB, a maximum current extracted from the USB port needs to be limited. Therefore, a current-limiting circuit needs to be integrated in a terminal device.
In the prior art, as shown in FIG. 1, a mainstream current-limiting circuit includes: an input voltage end, a precise current detecting resistor with a low resistance value, a VREF (voltage reference) module with an internal relative input voltage, a P-type MOS (Metal-Oxide-Semiconductor, metal oxide semiconductor) transistor, an output voltage end, an output capacitor, and an OP (Operational Amplifier, operational amplifier) with a low offset voltage. One end of the precise current detecting resistor with a low resistance value is connected to the VREF module and the input voltage end, and the other end thereof is connected to an inverting input end of the OP. One end of the VREF module is connected to one end of the precise current detecting resistor with a low resistance value, and the other end thereof is connected to a non-inverting input end of the OP. A gate electrode of the P-type MOS transistor is connected to an output end of the OP, a drain electrode is connected to the other end of the precise current detecting resistor with a low resistance value, and a source electrode is connected to the output capacitor and the output voltage end. As detecting resistors are connected in series in a current loop, a resistance value is generally 10 mohm-100 mohm.
The current-limiting circuit needs at least one external precise current detecting resistor with a low resistance value, and a range of the resistance value is 10 mohm-100 mohm for efficiency purposes. This type of detecting resistors is relatively large in size and high in price, and occupies a large PCB (Printed Circuit Board, printed circuit board) area, thereby increasing costs of the current-limiting circuit.